a. Field of the Invention
The invention relates to electrolytes and organic solvents for electrochemical cells. In particular, the invention relates to lithium-ion electrolytes and organic solvents for lithium-ion cells.
b. Background Art
Lithium-ion (“Li-ion”) cells typically include a carbon (e.g., coke or graphite) anode intercalated with lithium ions to form LixC; an electrolyte consisting of a lithium salt dissolved in one or more organic solvents; and a cathode made of an electrochemically active material, typically an insertion compound, such as LiCoO2. During cell discharge, lithium ions pass from the carbon anode, through the electrolyte to the cathode, where the ions are taken up with the simultaneous release of electrical energy. During cell recharge, lithium ions are transferred back to the anode, where they reintercalate into the carbon matrix.
Future NASA missions aimed at exploring Mars, the Moon, and the outer planets require rechargeable batteries that can operate effectively over a wide temperature range (−60° C. (Celsius) to +60° C. (Celsius)) to satisfy the requirements of various applications, including: Landers (lander spacecraft), Rovers (surface rover spacecraft), and Penetraters (surface penetrator spacecraft). Some future applications typically will require high specific energy batteries that can operate at very low temperatures, while still providing adequate performance and stability at higher temperatures. In addition, many of these applications envisioned by the ESRT (Exploration Systems Research and Technology) program will require improved safety, due to their use by humans. Lithium-ion rechargeable batteries have the demonstrated characteristics of high energy density, high voltage, and excellent cycle life. Currently, the state-of-the-art lithium-ion system has been demonstrated to operate over a wide range of temperatures (−40° C. to +40° C.), however, abuse conditions such as being exposed to high temperature, overcharge, and external shorting, can often lead to cell rupture and fire. The nature of the electrolyte can greatly affect the propensity of the cell/battery to catch fire, given the flammability of the organic solvents used within. Therefore, extensive effort has been devoted recently to developing non-flammable electrolytes to reduce the flammability of the cell/battery.
Desired properties for Li-ion electrolytes can include high conductivity over a wide temperature range (e.g., 1 mS (milli-Siemens) cm−1 from −60° C. to +60° C.); good electrochemical stability over a wide voltage range (e.g., 0 to 4.5V (volts)) with minimal oxidative degradation of solvents/salts; good chemical stability; good compatibility with a chosen electrode couple, including good SEI (solid electrolyte interface) characteristics on the electrode and facile lithium intercalation/de-intercalation kinetics; good thermal stability; good low temperature performance throughout the life of the cell, including good resilience to high temperature exposure and minimal impedance build-up with cycling and/or storage; and low toxicity. Since the flammability of the electrolyte solution in Li-ion batteries is a major concern, significant research has been devoted to developing electrolyte formulations with increased safety. Known electrolytes used in state-of-the-art Li-ion cells have typically comprised binary mixtures of organic solvents, for example, high proportions of ethylene carbonate, propylene carbonate or dimethyl carbonate, within which is dispersed a lithium salt, such as lithium hexafluorophosphate (LiPF6). Examples may include 1.0 M (molar) LiPF6 in a 50:50 mixture of ethylene carbonate/dimethyl carbonate, or ethylene carbonate/diethyl carbonate. More recently, electrolytes have also been developed which combine more than two solvents and/or have incorporated the use of electrolyte additives to address specific performance goals.
Fluorinated esters have been incorporated into multi-component electrolyte formulations and their performance was demonstrated over a wide temperature range (−60° C. to +60° C.) (see U.S. application Ser. No. 12/419,473 filed Apr. 7, 2009, for “Lithium Ion Electrolytes and Lithium Ion Cells with Good Low Temperature Performance”, Smart et al.). The fluorinated ester co-solvents were employed due to their favorable properties and improved safety characteristics, mainly associated with their low flammability associated with their halogenated nature. Since the flammability of the electrolyte solution in Li-ion batteries is a major concern, significant research has been devoted to developing electrolyte formulations with increased safety. To achieve this, a number of approaches have been adopted, including the use of low-flammability solvents and the use of electrolyte additives. Regarding the first approach, the use of halogenated solvents (Smart, et al., “Improved Performance of Lithium Ion Cells with the use of Fluorinated Carbonate-Based Electrolytes”, Journal of Power Sources, 119-121, 359-367 (2003)) and ionic liquids (Xu, Kang, “Nonaqueous Liquid Electrolytes for Lithium-Based Rechargeable Batteries”, Chemical Reviews, 104(10), 4303-4417 (2004)) have been pursued. With respect to the use of electrolyte additives, the main focus has been upon the use of phosphorus containing additives, including trimethyl phosphate (Wang, et al., “Nonflammable Trimethyl Phosphate Solvent-Containing Electrolytes for Lithium-Ion Batteries”, Journal of the Electrochemical Society, 148(10), A1058-A1065 (2001); Wang, et al., “Nonflammable Trimethyl Phosphate Solvent-Containing Electrolytes for Lithium-Ion Batteries”, Journal of the Electrochemical Society, 148(10), A1066-A1071 (2001)), triethyl phosphate (Xu, et al., “An Attempt to Formulate Nonflammable Lithium Ion Electrolytes with Alkyl Phosphates and Phosphazenes”, Journal of the Electrochemical Society, 149(5), A622-A626 (2002)), triphenyl phosphate (Doughty, et al., “Effects of Additives on Thermal Stability of Li-ion Cells”, Journal of Power Sources, Vol. 146, Issues 1-2, pp. 116-120 (2005)), tris(2,2,2-trifluoroethyl) phosphate (Xu, et al., “Nonflammable Electrolytes for Li-Ion Batteries Based on a Fluorinated Phosphate”, Journal of the Electrochemical Society, 149(8), A1079-A1082 (2002); Xu, et al., “Evaluation of Fluorinated Alkyl Phosphates as Flame Retardants in Electrolytes for Li-Ion Batteries”, Journal of the Electrochemical Society, 150(2), A161-A169 (2003)), and bis(2,2,2-trifluoroethyl) methyl phosphonate (TFMPo) (Xu, et al., “Nonflammable Electrolytes for Li-Ion Batteries Based on a Fluorinated Phosphate”, Journal of the Electrochemical Society, 149(8), A1079-A1082 (2002); Xu, et al., “Evaluation of Fluorinated Alkyl Phosphates as Flame Retardants in Electrolytes for Li-Ion Batteries”, Journal of the Electrochemical Society, 150(2), A161-A169 (2003)).
In addition, known improvements have been made to the safety characteristics of Li-ion electrolytes by the addition of flame retardant additives, such as triphenyl phosphate (referred to as TPhPh or TPP or TPPa), tributyl phosphate (referred to as TBP or TBuPh), triethyl phosphate (referred to as TEP or TEtPh), and bis(2,2,2-trifluoroethyl) methyl phosphonate (referred to as BTFEMP or TFMPo) (see NPO-46262, May 8, 2008). A number of electrolytes based upon these approaches have delivered good performance over a wide temperature range, good cycle life characteristics, and improved safety characteristics, namely reduced flammability. Since the flammability of the electrolyte solution in Li-ion batteries is a major concern, significant research has been devoted to developing electrolyte formulations with increased safety. To achieve this, a number of approaches have been adopted, including the use of low-flammability solvents and the use of electrolyte additives. As discussed above, regarding the first approach, the use of halogenated solvents (Smart, et al., “Improved Performance and Safety of Lithium Ion Cells with the Use of Fluorinated Carbonate-Based Electrolytes”, Journal of Power Sources, 119-12, 359-367 (2003)) and ionic liquids (Xu, Kang, “Nonaqueous Liquid Electrolytes for Lithium-Based Rechargeable Batteries”, Chemical Review, 104(10), 4303-4417 (2004)) have been pursued. With respect to the use of electrolyte additives, the main focus has been upon the use of phosphorus containing additives, including trimethyl phosphate, triethyl phosphate, triphenyl phosphate, tris(2,2,2-trifluoroethyl) phosphate, and bis(2,2,2-trifluoroethyl) methyl phosphonate (TFMPo) (see all above). In addition, known phosphorus based flame retardant additives have been investigated. Some of these electrolyte additives have been demonstrated to perform well in multi-component Li-ion battery electrolytes. For example, tris(2,2,2-trifluoroethyl) phosphate (see FIG. 5e) has been used in electrolyte solutions consisting of 1.0M LiPF6 in EC+EMC (1:1 wt %) in varying concentrations (Xu, et al., “Nonflammable Electrolytes for Li-Ion Batteries Based on a Fluorinated Phosphate”, Journal of the Electrochemical Society, 149(8), A1079-A1082 (2002); Xu, et al., “Evaluation of Fluorinated Alkyl Phosphates as Flame Retardants in Electrolytes for Li-Ion Batteries”, Journal of the Electrochemical Society, 150(2), A161-A169 (2003)). Tris(2,2,2-trifluoroethyl) phosphite (see FIG. 5f) has been used in concentrations of up to 15% in solutions of 1.0M LiPF6 PC+EC+EMC (3:3:4 wt %) (Zhang, et al., “Tris(2,2,2-trifluoroethyl) Phosphite as a Co-Solvent for Nonflammable Electrolytes in Li-Ion Batteries”, Journal of Power Sources, 113 (1), 166-172 (2003)), while other have investigated in 1.15M LiPF6 in EC+EMC (3:7 vol %) (Nam, et al., “Diphenyloctyl Phosphate and tris(2,2,2-trifluoroethyl) Phosphite as Flame-Retardant Additives for Li-ion Cell Electrolytes at Elevated Temperature”, Journal of Power Sources, 180 (1), 561-567 (2008)). Triphenylphosphite (see FIG. 5g) has been investigated in solutions consisting of 1.0M LiPF6 in EC+DEC+DMC (1:1:1 wt %) using concentrations of 10 wt % FRA (Ma, et al., “A Phosphorous Additive for Lithium-Ion Batteries”, Electrochemical and Solid State Letters, 11(8), A129-A131 (2008)).
Accordingly, there is a need for lithium-ion electrolytes containing flame retardant additives having increased and improved safety characteristics.